We discussed this in an anthropology class I took in college. It is a real fight for land, but it's also very formal and traditional affair. A time and place is agreed upon by both sides beforehand. Only traditional weapons like bows and arrows, spears, and slings (one can be seen here can be used. In a way it is like a sport... where some of the players die).

As I understand, the skirmishes ends when the head of one of the tribes decides too many of his men have been injured or killed, sends out a call, and his tribe leave the field. The other side then celebrates over winning the fight, as opposed to chasing down the retreating tribe and slaughtering everyone. It's relatively civil as far as warfare goes.

If I recall correctly, isn't this the same scheme that the ancient Greek city-states used? That is, the warring factions will decide meet one afternoon in a field, engage in battle, and then at a certain point a winner is proclaimed or something like that?